Unbeliever elected by Castro (7)
I believe the answer is:
infidel
'unbeliever' is the definition.
(thesaurus)
'elected by castro' is the wordplay.
'elected' becomes 'in'.
'by' says to put letters next to each other.
'castro' becomes 'fidel' (Fidel Castro famous ruler of Cuba).
'in'+'fidel'='INFIDEL'
(Other definitions for infidel that I've seen before include "Person who has no faith, or the wrong one" , "Person rejecting a specific religion" , "I don't believe" , "Person who does not believe in a religion" , "Person not operating in good faith" .)